3.5 sorcerer familiar

[jdizzlean]

when you multi class, or go into a PRC and it says the proverbial "+1 to existing spellcasting" and then in the text it states you get spells per day/ spells known and no other benefits... does that mean that your familiar doesn't level up, or IIRC is your familiar based solely off your HD, and so after gaining one, it doesn't matter what class you go into for it to advance?

[Crake]

The familiar gets your base saves, skill ranks, bab, half your hitpoints and your HD as effective hit dice for the purposes of adjudicating the results of effects based on HD. Those are the only things that scale directly with your level. The other things, including it's intelligence and special abilities all scale directly with sorcerer (or wizard) level. If you prestige out, your familiar only continues to accrue the things I mentioned earlier.

This is actually why many people will trade their base familiar for an ACF and pick up the obtain familiar feat, which grants you a familiar that scales the special abilities based on caster level, rather than class level.